Resources (oasis) renewal
There should be a way to renew your oasises, for example instead of having to grab a different oasis every time it's level is reduced a player can instead spend x amount of resources to get it to it's original level (level it was obtained by said player) or have a tech that reduces decay (usage) of oasises and it's top level can be never reduces level (lvl 1 10%, lvl 2 20%, lvl 10 stays at lvl obtained forever etc)